Abstract

A relativistic electron beam with a finite transverse dc momentum (β⊥=1/γ0) passing through a region of combined uniform solenoidal and longitudinal wiggler magnetic fields is observed to convert 25% of its kinetic energy into coherent radiation at frequency ω=γ20(kwV0+Ωc/γ0) if the phase velocity of the generated wave is slightly above the speed of light. In this situation, the bunchings of the slow electron‐cyclotron mode and free‐electron laser modes with combined solenoidal and longitudinal wiggler fields (lowbitron) are observed to compensate each other, which gives rise to a finite threshold for lowbitron operation. In order to attain high efficiency, the wiggler strength of a lowbitron must substantially exceed the threshold.